To determine which of the given expressions is rational, let's analyze each one step-by-step.
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\( \sqrt{3} + \sqrt{3} \): This simplifies to \( 2\sqrt{3} \), which is irrational since \( \sqrt{3} \) is irrational.
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\( \frac{1}{\sqrt{3}} \): This is also irrational, but if we rationalize the denominator, we multiply by \( \frac{\sqrt{3}}{\sqrt{3}} \), which gives us \( \frac{\sqrt{3}}{3} \). This expression is still irrational.
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\( \sqrt{3} \times \sqrt{3} \): This gives us \( 3 \), which is a rational number.
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\( \sqrt{3} \cdot 1 \): This is simply \( \sqrt{3} \), which is irrational.
Based on the analysis, the only rational expression is \( \sqrt{3} \times \sqrt{3} = 3 \). So the correct answer is:
3 (the product of \( \sqrt{3} \) with itself).